1. Field of the Invention
The present invention relates to a motor for a disc drive and more particularly to a motor for a disc drive for driving a recording medium in the form of a disc so as to write and read data onto and out of the disc.
2. Description of the Prior Art
A typical example of a disc driving device of the type described above is a floppy disc drive unit (to be referred to as "an FDD" hereinafter in this specification) for writing and reading data onto and out of a floppy disc which is a magnetic recording medium.
Especially, as a disc driving motor of a driving source for rotating a disc in a 3.5"-FDD, a so-called surfaces opposing type motor is used. As shown in FIG. 12, the surfaces opposing type motor comprises a disc-shaped rotor yoke 32 whose center coincides with that of a spindle shaft 30 and on the cylindrical peripheral surface thereof is mounted a driving magnet 31 disposed in parallel and opposing relationship with a projected type core yoke 34 wound with a coil 33. In the 3.5"-FDD of the type using the above-mentioned motor, it is impossible, if a 3.5"-FDD thin structure is to be obtained to stack a magnetic head mounted on a head carriage on the motor in the vertical direction so that they are disposed on the same plane in a closely-spaced apart relationship. As a result, there arises a problem of leakage flux from the driving magnet 31 mounted on the outer peripheral surface of the rotor yoke 32 because it is close closer to a magnetic head.
In order to solve this problem and to cope with the magnetic saturation of the rotor yoke 32, the thickness of the entire rotor yoke 32 has been increased so as to prevent the magnetic saturation thereof and the adverse effect of the magnetic flux from the driving magnet 31 on the magnetic head.
However, there arises another problem that the thicker the whole rotor yoke 32 is made, the thicker the FDD becomes.
Furthermore, the thickness of the FDD is also increased for the following reason. In order to securely maintain the core yoke 34 in a stationary state, a screw through hole 35 (not internally threaded) is extended from a surface (lower surface) of the core yoke 34 which is not in opposing relationship with the rotor yoke 32 to a surface (upper surface) of the core yoke 34 which is in opposing relationship with the rotor yoke 32. A small screw is extended through one of the component members of the motor and the screw hole 35 from the lower surface to the upper surface of the core yoke 34 beyond the latter and a nut (not shown) is threadably engaged with the screw, whereby the core yoke 34 is securely fixed. The core yoke 34 is a laminated body consisting of a plurality of ferromagnetic sheets or the like laminated one upon another.
However, in order to make FDDs still thinner, a construction has been recently proposed and used in which a driving pin 36 of a chucking mechanism for holding a disc which engages the disc and transmits the rotating force thereto is vertically movably mounted on the rotor yoke 32 of the disc driving motor. With this construction, in order to prevent the vertically movable driving pin 36 on the side of the rotor yoke 32 from colliding with the core yoke 34, a space must be provided between the rotor yoke 32 and the core yoke 34. When the upper end portion of the screw and also the nut are projected beyond the upper surface of the core yoke 34, the height of the space must be increased so as to avoid collision of the driving pin 36 against the screw or the nut projecting beyond the upper surface of the core yoke. Thus, this construction becomes a problem which must be solved before making the FDDs still thinner.
A further problem encountered is that it takes a relatively large number of assembly steps, including the step for securely maintaining the laminated body, because the core yoke 34 is in the form of a laminated body.